This invention relates to a pressure-resistant, sliding tetrafluoroethylene resin composition used as material for a seal ring, and a seal device to be brought into sliding contact with an aluminum metal.
A material molded from a composition composed mainly of tetrafluoroethylene resin (hereinafter abbreviated to "PTFE") is used widely as material for sliding seal devices such as seal rings due to its low elastic modulus, ease of handling, stable sealing properties, and low and stable dynamic friction coefficient.
According to the environment in use and the material of the mating member, such sliding seal devices molded from PTFE may contain various additives, typically glass fiber, carbon fiber, graphite, mica and talc.
For example, unexamined Japanese patent publication 1-233150 proposes to mix a thermoplastic polyimide in PTFE, the main component, to improve the creep resistance of PTFE and further add glass fiber, glass beads or graphite to the mixture.
Various similar attempts to improve the creep resistance of PTFE have been made. Such attempts include adding a tetrafluoroethylene-fluoroalkylvinylether copolymer (PFA) or a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), which are both superior in creep resistance to PTFE. But it only slightly improved the creep resistance. It failed to improve the creep resistance of PTFE to a level comparable to that of PFA and FEP.
It is also possible to increase the elastic modulus and improve, to a certain degree, the creep resistance by adding a fibrous filler such as glass fiber or carbon fiber, and a flaky filler such as graphite, mica or talc to a main component consisting of a partially modified tetrafluoroethylene copolymer, alias modified PTFE resin, which is a copolymer of tetrafluoroethylene and partially modified tetrafluoroethylene (unexamined Japanese patent publication 5-239440).
If a seal device made from such a partially modified PTFE resin composition containing the above-described fillers is brought into sliding contact with a mating member made from an aluminum alloy under high contact pressure, the sliding surface of the mating member tends to be damaged and worn abnormally. Trials were made to reduce the contents of fillers to avoid this problem. But it resulted in a lowering of the creep resistance and wear resistance.
Aromatic polyamide fibers (aramide fibers) will not damage an aluminum alloy member, but they have no effect in improving the creep resistance. Unexamined Japanese patent publication 6-184385 proposes to add whiskers that are low in Mohs hardness to a resin to prevent sliding wear of an aluminum alloy member. But such whiskers cannot sufficiently reinforce the resin, so that the resin composition tends to suffer heavy creep deformation and wear when brought into sliding contact with a mating member at high temperature and high contact pressure.
The properties required for apparatuses having sliding seal devices such as seal rings include compactness, high performance and smooth sliding of the seal device relative to a mating member even under high pressure.
Specifically, in a heat engine that uses a substitute freon as its refrigerant, the vapor pressure of the refrigerant was 1 MPa to 1.7 MPa at normal temperature and 5 MPa at temperatures higher than 100.degree. C. The water pressure in the pipes leading from a water tank on the rooftop of a high-rise building can reach as high as a few times the water pressure in similar pipes of low buildings. Hydraulic systems of today's automobiles are subjected to larger loads than ever before. In particular, due to recent relaxation of the regulation on vehicle weights, the required contact pressure of the seal devices in hydraulic power steering units in today's trucks can reach as high as more than 14 MPa.
But seal rings and other seal devices made from PTFE resins having conventional compositions such as described above are too large in creep deformation to exhibit required sealability under high pressure. They also tend to be worn abnormally due to creep deformation in a shearing direction.
In hydraulic units for trucks and buses, seal devices are undoubtedly subjected to such creep deformation because sealability is required at high temperatures around 100.degree. C.
For sealing devices such as seal rings mounted in hydraulic cylinders, either the shaft or the cylinder is formed from an aluminum alloy for easy machining and reduced weight. In such a seal device, the aluminum surface, which slides in contact with lubricating oil, is more likely to be damaged than a surface that slides in a non-lubricated (dry friction) state.
This is because lubricants in the PTFE composition do not transfer to the mating member, as is usually the case with a dry frictional surface, if no sufficient lubricating oil is supplied to the sliding surfaces due to high contact pressure or if for some unknown reason, an extremely thin film of oil is formed on the sliding surfaces. If this happens, neither solid nor liquid lubricants can be supplied onto the sliding surfaces, thus causing wear damage to the aluminum metal.
With a compressor for compressing various gases such as a refrigerant as a substitute for fleon, air, helium and natural gas, even if it is intended to be used at normal temperature under a contact pressure below 10 MPa, sliding surface temperature and atmospheric temperature may rise to over 100.degree. C. due to frictional heat of unlubricated seals. In such a case, the unlubricated seals may fail to reveal sufficient sealing properties due to creep deformation.
Especially if the mating members to be brought into sliding contact with the seals are made of a stainless steel having a low thermal conductivity, the sliding surface temperature and atmospheric temperature tend to rise excessively due to heat accumulation by the stainless steel though the lubricative substance transfers to the mating member. If the mating members are made of an aluminum alloy having a corrosion-resistant coating, no transfer occurs because the surface roughness is small. Thus, not only the coating but the aluminum alloy substrate may be damaged.
An object of this invention is to provide a pressure-resistant, sliding tetrafluoroethylene resin composition which exhibits a good sealability while keeping creep deformation to a minimum when brought into sliding contact with a mating member at a high surface pressure of over 10 MPa, and which will not damage a mating member made from an aluminum alloy while sliding in contact with lubricating oil.
Another object of this invention is to provide a tetrafluoroethylene resin composition which is small in creep deformation under unlubricated friction under a contact pressure not less than 4 MPa at normal temperature so that it can reveal sufficient sealing properties, and which will not damage the sliding surface even when brought into sliding contact with an aluminum alloy having a coating or a stainless steel.